This invention relates to a heating calender roll, particularly for the production of packing sheets of rubber-bonded asbestos fibers ("It" packing sheets). It is known to produce such sheets on a two-roll calender comprising a heating roll and a cooling roll. A mixture of asbestos and rubber, having a consistence ranging from that of paste to that of a crumbly mass, is fed into the nip between the roll, with uniform distribution, and is rolled at high pressure, into a thin layer. The cooling roll, which generally is smaller, is generally used as an idler, the larger and heated roll serves as driving means. The mixture is gradually built up on this roll in spiral form to provide thin layers, and is at the same time vulcanized. The vapors of solvents liberated incident to the heating up of the mass are sucked off from the calender. When the building up and vulcanization of the sheet has been completed on the heating roll the finished sheet is divided in a direction parallel to the axis of this roll and is drawn off. A finished sheet has a width equal to the length of the roll and has a length equal to the diameter of the roll.
For reasons for economy it is generally preferred to make the sheets as large as possible. This in turn calls for a correspondingly large heating roll. Conventional rolls of this kind often have a diameter of 1000 to 2000 mm and a width of 1600 to 3100 mm, corresponding to sheet dimensions which range between 1500 .times. 3000 mm and 3000 .times. 6000 mm.
Heretofore it was usual to support the shell of the heating roll (usually made of a hard material such as chilled cast iron in order to withstand the extremely high specific forces encountered during the calendering) on dome-shaped end structures flange connected to the shells; to introduce the heating medium (usually saturated steam at pressures up to 16 atmospheres) into the interior of the roll defined by the shell and two end structures; and in order to withstand the high steam pressures, to make the end structures of cast steel. Hollow trunnions have been formed on the central portions of the dome shaped end structures to support the roll on a hollow shaft (conventionally made of cast steel and arranged for the passage of heating steam into the interior of the roll and the withdrawal of condensate from this interior).
These former constructions of heating rolls have a number of drawbacks, connected with problems of heat flow through the various elements and of resulting mechanical forces. In particular:
The time required for bringing the roll to the proper operating temperature was extremely long because of the great weight and mass of the end structures as well as the shell. The roll had extremely high thermal inertia when reacting to impulses for changes of surface temperature. In addition the temperature gradient between inner and outer surfaces of the shell was high, as where the corresponding tensions in the shell and the resulting dangers of shell fracture. The wear and tear of gasket elements between the shell and the end structures was very considerable, under the rotation of the roll and the successive changes of thermal and mechanical loads on these elements. Since the cast steel of the end structures has a higher coefficient of thermal expansion than the chilled cast iron of the shell the terminal zones of the shell were exposed to greater changes of diameter than were the intermediate zones of the shell. This fact made it necessary to grind the terminal zones in slightly conical form when cold, in order to provide cylindrical form of the entire shell surface when hot, as is required for proper calendering. This requirement or necessity in turn involved much delay and cost, and could be fulfilled only by highly experienced technicians.
Still further it was necessary to have the heating roll tested and approved, for safety, as such a roll, containing a large volume of steam under high pressure, in effect was a steam kettle. The required tests were additional elements of delay and expense.